The invention starts from machines to which a continuous web is fed to be provided with weld or seal lines at a welding or sealing station and, at a possible perforation station, be provided with transverse perforation lines. Such a machine, which is intended for making plastic bags, is disclosed in, for instance, U.S. Pat. No. 4,950,217.
The welding station in this bag-making machine comprises two opposite, transverse welding units, between which the continuous web travels. Each welding unit comprises at least one welding or sealing jaw, which is driven to circulate in an endless loop via a drive means in the form of a chain transmission. The drive means of the opposite welding units are so synchronized that the sealing jaws during their movement are made to abut against each other while clamping the web traveling through the machine. This clamping is provided by the sealing jaws during their circulating movement passing from a curved portion of the loop to a straight portion. After the abutment, the sealing jaws move along the straight portion in the feeding direction of the web while abutting against each other, to form weld lines in the web. The distance along which the sealing jaws abut against each other is the so-called welding distance.
In the construction above, the drive means consist of chains. The chain transmission results in undesired noise levels and places demands on maintenance, such as lubrication. The lubrication may result in dirt and oil splashes depositing in the machine.
In a bag-making machine as stated above, the speed of the sealing jaws should also be adjusted to the speed of the web along the entire welding distance. The driving chains are normally designed in such a manner that the speed of the sealing jaws along the straight portion corresponds to the speed of the web through the machine. Here problems may arise when the sealing jaws are made to abut against each other, since this abutment takes place before the straight portion, i.e. when the sealing jaws move along a portion in the form of a circular arc and thus have a higher peripheral speed than the web. The abutment of the sealing jaws produces a jerk in the web, which runs the risk of being torn off. This problem can be obviated by the speeds of the drive means being lowered momentarily during abutting of the sealing jaws. However, this solution makes great demands on drive and transmission, and therefore the construction will be both complicated and costly.
In attempts at solving the above-mentioned problem, belt operation has been used, in which a sealing jaw is fixed directly to a rubber block which is fixed by vulcanization to a driving belt. However, it has been found that the rubber block was subjected to great torsional forces while the sealing jaw moved along a circular arc. These torsional forces were so great that gradually the rubber block was broken away from the driving belt.